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Sugar
partitioning is general definition of assimilate allocation from source to sink
organs while sucrose is distributed over the long distance through phloem sieve
tubes towards heterotrophic organs. Sucrose transporters regulating the efflux
of sucrose into the apoplast of the source and sink cells. They belong to
integral cell membrane proteins encoded by numerous genes in plants. Recent
insights on molecular aspects of transporters underscore the crucial role of
sucrose transporters in different plant biological phenomena. Some
investigations with regard to potential role of sucrose transporters on sugar
partitioning and consequent stimulated responses of plants are discussed in
current review article.
INTRODUCTION
Sucrose is the end product of photosynthetic
process and, in most plants; it is the predominant form of carbon transported
to the heterotrophic tissues [1-3]. Sugar partitioning or in a more meaningful
definition, sucrose allocation between sink-source tissues is a fundamental
process in all plants. More than 80% of the photosynthetic carbon product is
transported via the plant’s phloem vascular system to heterotrophic organs of
plants [4]. This transport system comprises three steps: (i) loading of photosynthates
into the sieve element companion cell complex (se-cc complex) of minor veins in
exporting leaves, (ii) translocation from source to sink, and (iii) unloading
in growing or storing sinks [5]. The movements of most solutes through the
membrane are mediated by membrane transport proteins which are
specialized to varying degrees in the transport of specific molecules. Sucrose
transporters (SUT) are structurally integral proteins in the cell membrane
mediating sucrose movement in or out of the cell. The transport is active and
has been described as a sucrose-proton co-transport with a 1:1 stoichiometry
[6]. Sucrose transporter function in vacuole have been characterised as
sucrose/H+ symporters and efflux of sucrose into source or sink apoplast is
supported by sucrose/H+ anti-porters [7]. From a structural point of view,
transporters are highly hydrophobic proteins and belong to the class of
metabolite transporters consisting of two sets of six membrane spanning
regions, separated by a central cytoplasmic loop [8].
SUCROSE TRANSPORTERS GENE FAMILIES
Using expression
profiling, the role of OsSUT1 during germination and early growth of rice
seedlings has been examined in detail. This gene was present in the companion
cell’s sieve elements of the scutellar vascular bundle, where it may play a
role in phloem loading of sucrose for transport to developing shoot and roots.
OsSUT1 was also present in the coleoptiles and the first and second leaf blades
and phloem of primary roots [22]. In another study, twenty-six suppression
lines were created using an antisense construct containing a portion of the
3’-coding and non-coding regions of OsSUT1 driven by the maize ubiquitin-1
promoter. The functional analysis of OsSUT1 antisense lines revealed the
important role of this OsSUT1 gene on germination, growth and grain filling of
rice plants [23]. With the aim of investigating the role of OsSUT1 in the
transport of assimilates along the entire long-distance pathway, an experiment
using a promoter::GUS (ß-glucuronidase) reporter gene and immunolocalization of
OsSUT1 proteins was conducted in rice. The results revealed that the mature
phloem of vegetative tissue is involved in long distance assimilate transport
pathway during grain filling. It was proposed that OsSUT1 may play a primarily
role in the phloem loading of sucrose retrieved from the apoplast along the
transport pathway [24].
As previously explained, sucrose supply to terminal sink organs is crucial
for the energy status and the control of flowering, seed setting and filling
and final yield product [25,26]. Besides, tight regulation of sucrose
allocation is required to modulate carbon allocation in response to changing
environmental conditions. Sucrose transporters are tightly regulated at various
levels, allowing adaptation to external stimuli such as temperature, light
regime, photoperiod, pathogen attack or other stresses [7,27,28]. In a case
study the effects of sugar partitioning on salinity tolerance of rice antisense
lines was investigated [29] and an overview is presented in continuing.
SALINITY TOLERANCE
OF SUCROSE TRANSPORTERS ANTISENSE LINES
Sugar is depleted in the roots of sensitive rice cultivars after exposure
to salt stress. In order to investigate this phenomenon further, we took
advantage of reverse genetics by using sucrose transporter antisense lines to
modify sucrose allocation in rice plants. Studies were carried out with the
hypothesis that the knock-down of an acclimatory response involving a key
metabolite such as sucrose would decrease the performance of the plants.
Surprisingly, morpho-physiological evaluation showed that the transgenic lines
performed the same or better than the wild type under salt-stress. In addition,
during salt stress, antisense lines demonstrated higher sucrose,
glucose-6-phosphate and fructose-6-phosphate levels in the roots of transgenic
plants as compared to WT plants. To examine why this might occur, we first
measured the root starch contents of lines but could not see any reasonable
trend. We then profiled the expression levels of genes in the sucrose
transporter family in plants grown with or without salt, and detected a
significant increase in the expression of OsSUT2 and OsSUT4 genes
in antisense lines as compared to WT, which could explain higher maintenance of
sugars in the roots of antisense lines under salt stress. All these
observations support the hypothesis that the modification of sucrose allocation
toward the root upon salt stress improves the salinity tolerance of rice
cultivars [29].
To the best of our knowledge, the experiments that comparatively
investigate the metabolic phenotype as regards sugars in rice cultivars under
salt stress are rare. In this regard, the only study we know of was by Boriboonkaset
et al. [30] who
looked at the effect of exogenous sugar classes and concentrations on the
salt-tolerance of indica rice (Oryza
sativa L.) and concluded that exogenous sucrose and glucose in the root
play a direct role as a carbon source, improving salinity tolerance and
maintaining growth and development in rice cultivars. Therefore, not only is a
higher indigenous supply of sugars (mainly sucrose) to the root – as discovered
in current project – improve the salinity tolerance of rice cultivars, but also
an exogenous supply of sugars helps significantly. These issues strongly
support the hypothesis that sugar
maintenance in the root of rice plants upon salt stress improves the salinity
tolerance of lines. Based on the results a non-linear sugar
accumulation in the root of plants in response to salt stress was monitored. In
this regards, the cultivars showed an extreme sugar accumulation in
intermediate salt-acclimation. This promising
hypothesis has potential practical applications for plant breeders.
CONCLUSION
To sum up,
overexpressing and silencing of sucrose transporters genes provide strong
evidence of the importance of sucrose transporters role in the axial pathway
and terminal sink organs of heterotrophic tissues in different plants. Sugar
allocation to different organs is regulated by sucrose transporters. Therefore,
sucrose transporters play a major role in response of plants to environmental
stimuli, determining the seed setting and filling after flowering, influencing
pollen tube growth, assigning the process of tuberisation and controlling some
other developmental phenomena in plants. Recent insights on molecular aspects
of sugar partitioning and future perspectives of that, motivates the scientist
to modify targets involved in sucrose allocation in plants and test its
potential application to improve the plant performance.
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